This invention relates to manhole assemblies and particularly but not exclusively to a device for the rotational locking of a manhole cover to a manhole frame.
Roadway manholes are known which include a frame on which a cover is firmly secured by tightening means usually including lugs integral with the cover and co-operating with inclined ramps on the frame, rotation of the cover in the direction of tightening having the effect of moving the cover axially towards and into tight engagement with the frame.
It is necessary, in the case of such manholes to provide a device for locking the cover tightly in its final position so as to resist the frequent and large forces resulting from heavy vehicles running on the road. These forces may indeed lead to the cover becoming loosened and displaced from the frame.
Another problem presented by such roadway manhole assemblies is that when the covers are tightened up and locked on to the frame, it is impossible for a person accidentally shut in beneath the cover to unlock the cover